Pat chicken dry with paper towels and season with salt and pepper. Heat a drizzle of olive oil in a medium, heavy-based fry pan or pot (with a lid) on medium to high heat. Cook chicken for 3-4 minutes, until well browned. Remove from pot and set aside.
Add a drizzle of oil, bacon, shallots and mushrooms to pan. Cook until browned, 5-6 minutes. Add garlic and cook for a further minute then pour in brandy or Cognac (if using), scraping the bottom of the pan with a wooden spoon to release any brownings. Cook until most of the liquid has evaporated.
Return chicken to pan with any resting juices. Add wine, stock, tomato paste and bouquet garni, and season with salt and pepper. Cover, bring to a gentle boil, then reduce to a simmer and cook for 15-20 minutes until chicken is cooked through.
Bring a large pot of salted water to the boil and cook potatoes for 12-15 minutes until soft. Drain, return to pot with herbs and butter, and gently crush with a fork. Season to taste with salt and pepper.
Bring a second large pot of salted water to the boil and cook Brussels sprouts for 1-2 minutes, then drain. Allow to dry while you cook bacon. Heat a drizzle of oil in same pot on high heat. Cook bacon for 4-5 minutes until crispy. Add Brussels sprouts and cook for 2-3 minutes, until light golden. Season to taste with salt and pepper.
Remove bouquet garni from coq au vin and discard, then stir in cornflour mixture. Simmer for 1-2 minutes until sauce has thickened.
TO SERVE, divide crushed potatoes between plates, top with chicken, spoon over sauce and garnish with parsley. Serve with Brussels sprouts on the side.